The present invention relates to an intake port structure for an internal combustion engine having multiple cylinders, and more particularly to an intake port structure for improving a mixture of air with fuel.
Generally, in order to mix the air and the fuel for improving combustion efficiency, it is effective to produce a swirl of air-fuel mixture in a combustion chamber. There has been proposed to provide a cylinder head having two intake valves for one cylinder. For example, Japanese Patent Application Laid Open No. 58-160513 discloses a cylinder head having two intake ports in which a primary intake port is formed in a helical port and a secondary intake port is formed in a straight port. The length of the primary helical port is longer than that of the secondary straight port. Japanese Utility Model Application Laid Open No. 55-44004 discloses a cylinder head wherein a low-load intake port is bifurcated for adjacent cylinders. A diffuser is provided on an intake valve seat so as to generate a swirl.
However, since both the primary and secondary intake ports in the former prior art are connected to the same cylinder, the incoming air-fuel mixture flows separately into the cylinder through two intake ports. Thus, a strong swirl can not be generated in such a system.
The structure of the latter prior art is complicated because of providing the diffuser in the cylinder.